1. Field of the Invention
The invention relates to a filter circuit for filtering at least one sound carrier in a composite video signal by means of at least one signal filter having at least one filter frequency which is adjustable in dependence upon a control signal.
2. Description of the Related Art
A problem in such filter circuits is that the frequency of the sound carrier may assume different values within a range between approximately 4.5 and 6.5 MHz for composite video signals of different standards. The circuit should therefore be implemented in such a way that it can filter these different sound carrier frequencies. To this end, ceramic filters may be used between which switching takes place in dependence upon the transmission standard; however, such a circuit is very elaborate.
A filter circuit for a video signal whose frequency can be switched in dependence upon the presence of either a PAL or an NTSC video signal is known from U.S. Pat. No. 5,239,367. To this end, a reference filter is used which operates at a reference frequency and whose output signal is applied to a phase discriminator which also receives the reference signal at a further input. Dependent on the phase position of the two signals applied to the phase discriminator, the filter frequency of the reference filter is readjusted by means of the output signal of the phase discriminator in such a way that the filter is tuned to the reference frequency of the reference signal. In this state, the reference filter shifts the phase position of the reference signal applied thereto by such a value that no further readjustment takes place anymore. The signal with which the reference filter is readjusted is used for activating a switch which switches different current sources to the signal filter so that its filter frequency is switched.
Apart from the fact that the frequency position, of the signal filter of this circuit arrangement is switchable only between two frequency positions and that the circuit, after all, serves more for recognizing the PAL/NTSC standard and the corresponding switching of the signal filter, it has the additional drawback that the circuit can be optimized only for a given process state. By switching to different frequencies, the reference filter operates in other frequency positions than the signal filter. Furthermore, the overall circuit can only be optimized for a given temperature; by varying the chip temperature, spreads in the frequency position of the signal filter occur as before and may render the circuit unusable.
It is an object of the invention to provide a filter circuit of the type described in the opening paragraph, in which the signal filter operates at an adjustable nominal signal frequency, also while taking spreads in the manufacturing process of the circuit into account, independently of the temperature at which the circuit is operated and independently of the frequency position of the signal filter.
According to the invention, this object is achieved in that the filter circuit comprises a reference filter whose filter frequency is adjustable in dependence upon the control signal and which shifts the phase of a reference signal applied thereto by a defined value when tuning the filter frequency to the frequency of said reference signal, in that a phase comparator is provided which receives the output signal of the reference filter and the reference signal, in that the control signal is derived from the output signal of the phase comparator in such a way that the reference filter is tuned to the frequency of the reference signal, and in that an output signal of a controlled oscillator of a phase-locked loop is used as a reference signal which serves for demodulating a sound signal modulated on the sound carrier and whose controlled oscillator supplies an output signal at the frequency of the sound carrier in the locked state of the phase-locked loop.
The reference filter which receives the reference signal shifts this signal by a defined value when tuning the filter frequency to the frequency of this reference signal. The phase comparator receives both the phase-shifted reference signal and the original reference signal. The output signal of the phase comparator is used for readjusting the reference filter. The same control signal is also used for continuously readjusting the filter frequency of the signal filter. This signal is implemented in such a way that the reference filter is tuned to the frequency of the signal applied thereto. Thus, spreads, temperature fluctuations, etc., are stabilized and compensated by readjusting the reference filter. In a corresponding manner, the signal filter is also readjusted.
In the prior-art circuits, such a circuit can be optimized for only one frequency position. However, if the signal filter is to operate at various frequencies, which may deviate from the operating frequency of the reference filter, external effects influence the circuit as regards the ratio between the frequency of the signal filter and the frequency of the reference filter. Dependent on the operating frequency of the signal filter, these effects occur in different ways in the two filters so that the signal filter can no longer be readjusted optimally. Also, process spreads of temperature affect the frequency position of the signal filter; when the signal filter operates at a different frequency than the reference filter, these effects on the filters are also different so that an optimum readjustment is no longer possible.
For a substantial elimination of these drawbacks, the output signal of a controlled oscillator of a phase-locked loop is used as a reference signal. This phase-locked loop receives the composite video signal which is also applied to the signal filter of the filter circuit. This phase-locked loop serves for demodulating the sound signal modulated on the sound carrier. In its locked state, the phase-locked loop operates at the frequency of the sound carrier. The voltage-controlled oscillator of this phase-locked loop thus supplies a signal which exactly corresponds to the frequency of the sound carrier to be filtered in the composite video signal.
Since this signal is used as a reference signal for the reference filter, it is achieved that both the reference filter and the signal filter always operate at the same frequency, namely, at the frequency of the sound carrier to be filtered. The above-described effects, process spreads and temperature fluctuations thus influence the two filters in similar ways. Not only the readjustment of the reference filter then succeeds optimally under different external effects, but also that of the signal filter, because it operates at the same filter frequency.
As a result, it is achieved that the filter circuit always operates optimally also at different filter frequencies, i.e., the signal filter is always optimally adjustable at the frequency of the sound carrier to be filtered. Moreover, the advantage is obtained that it is no longer necessary to generate an additional switching signal for switching the signal filter.
These advantages may be even further enhanced in that, by building up the reference filter in the same technique as the signal filter, and by influencing the filter frequency of the reference filter by the control signal in the same way as the filter frequency of the signal filter, both the reference filter and the signal filter have the same technical structure so that process spreads, temperature fluctuations and other influences have the same effect on both filters.
The control signal may be generated from the output signal of the phase comparator such that two current sources, or one bidirectional current source, charge or discharge a capacitance with currents of different signs in dependence upon the output signal of the phase comparator, and in that a comparator is provided which supplies the voltage across the capacitance at a first input and receives a reference voltage at a second input, and supplies the control signal at the output, referring to a simple circuit for generating the control signal.
The phase-locked loop, from which the reference signal is obtained, may be preferably a narrow-band phase-locked loop, which has no amplitude limitation but an amplitude control and is implemented in such a way that it has its own selectivity.
For example, if no sound carrier were present in the composite video signal, the phase-locked loop could not lock at such a sound carrier. In this case, it is advantageous to maintain the frequency of the controlled oscillator of the phase-locked loop in a nominal frequency range by means of a fine-tuning circuit. It is thereby achieved for the filter circuit according to the invention that, in such a case, the signal filter is also maintained in a nominal frequency range.
A further embodiment of the invention, is characterized in that integrator filters are used for the signal filter and for the reference filter. Their integrator elements are advantageously built up in the same technique. Even when filters of different orders were used, the above-described influences on the filters would have the same effect.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.